The present invention relates to a high speed control circuit for power field effect transistors.
It applies to the construction of power supplies and chopping converters as well as to DC motor control devices.
The power transistors which are used in these applications must be able to switch rapidly, so as to reduce energy losses and increase the chopping frequency. The reduction of the energy losses increases the efficiency, simplifies the cooling systems and increases the reliability. Increasing the chopping frequency reduces the size and the price of the inductances of the transformers as well as of the capacitors used and increases the response speed of controlled systems.
However, the development of such devices is restrained by two main obstacles which limit the switching speed of the field effect transistors. This limiting factor is the high values of the input, gate-source C.sub.GS and drain-gate C.sub.DG capacities. The first capacitance (C.sub.GS) is caused by a simple capacitive effective. The second is caused by Miller effect. These two capacities increase the rise times of the control signals applied to the gate inputs of the field effect transistors, and determine the time required for passing from the voltage V.sub.GS(off) for which the transistor is disabled to the voltage V.sub.GS(on) for which it is enabled.
To reduce the opening or closing time of a power field effect transistor, one known solution consists in increasing the current flowing through the gate of the transistor. It requires inserting a current amplifier with a high switching speed between the control circuit and the gate of the power transistor. However, this solution meets some matching difficulties when in ceertain constructions a galvanic insulation must be inserted between the control circuit and the gate. This situation is met with more particularly when, for a static DC power supply, for example, the configuration of the chopper is such that the source of one or more power transistors is brought to a high or variable potential or when, for safety reasons, it is necessary to isolate the chopper circuit from the mains by a photocoupler or by a transformer.
Although the photocoupler allows the signal to be transmitted in the information state, it has the disadvantage of having to be followed by a high speed amplifier fed by an energy source itself galvanically isolated, which complicates this type of construction.
A transformer, on the other hand, overcomes this drawback, but in order to be efficient it must have a reduced leakage inductance so as to have a response compatible with the control time of a few nanoseconds for power field effect transistors, which makes this type of construction expensive.